Is there a way to see what my code looks like after type erasure?

user_4567552 0
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I would like to know what tool can be used to view code after type erasure. For example, I'd like to see what the following code looks like after type erasure. I know that generic type information is stripped prior to final compilation, replacing type references with Object or boundary types, creating bridge methods, etc. I want to see the source code after the erasure process takes place.

  public static void main(String[] args) {
    Predicate<Object> objPredicate = (Object c) -> c.toString().length() > 2 ;
    Predicate<? super Integer> predicate = objPredicate;
    Number n = Integer.valueOf(22);
    System.out.println(predicate.test(n)); //this line does not compile
  }

[If compiling code is required to see an erasure inspection tool work on the example, the non-compiling line can be omitted or n can be cast to Integer]

generics java
3 Answers

I know that generic type information is stripped prior to final compilation

This is not quite true, as erasure is a part of the compilation process itself, not something that happens prior to compilation. i.e. erasure is not a transformation that happens on source code.

As such, there is no way (that I know of) to programmatically get a transformed version of the source code, but we can figure out what happens for simple examples like this by following the rules in JLS 4.6:

Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows:

  • The erasure of a parameterized type (§4.5) G<T1,...,Tn> is |G|.

  • The erasure of a nested type T.C is |T|.C.

  • The erasure of an array type T[] is |T|[].

  • The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

  • The erasure of every other type is the type itself.

Type erasure also maps the signature (§8.4.2) of a constructor or method to a signature that has no parameterized types or type variables. The erasure of a constructor or method signature s is a signature consisting of the same name as s and the erasures of all the formal parameter types given in s.

The return type of a method (§8.4.5) and the type parameters of a generic method or constructor (§8.4.4, §8.8.4) also undergo erasure if the method or constructor's signature is erased.

The erasure of the signature of a generic method has no type parameters.

For your code snippet we get (ignoring trivial erasures):

Predicate<Object> -> Predicate
Predicate<? super Integer> -> Predicate

Yielding:

public static void main(String[] args) {
    Predicate objPredicate = (Object c) -> c.toString().length() > 2 ;
    Predicate predicate = objPredicate;
    Number n = Integer.valueOf(22);
    System.out.println(predicate.test(n)); //this line does not compile
}

Furthermore, the erased signature of the Predicate::test method is:

boolean test​(Object t)
user_36991390

If you have a very simple type like :

public static void main(String[] args) {
    Predicate<? super Integer> predicate = x -> x > 1;
}

it gets compiled with invokedynamic, like:

invokedynamic #7,  0  // InvokeDynamic #0:test:()Ljava/util/function/Predicate;

and if you look at the BootstrapMethods, the MethodType preserves (almost) all generic information (I don't think it can preserve the fact that there was a wildcard in use):

#41 (Ljava/lang/Integer;)Z

This reads as "takes an Integer, returns a boolean". So, in theory, it could be possible to re-construct the generic information back. I have tried Intellij to reconstruct the types from a .class file - and it does not preserve the generic types. I am not aware of a tool that would do that.

user_10593720

On github, I found the java-based open source gui tool bytecode viewer which did the job as follows:

 public static void main(String[] var0) {
      Predicate var1 = (var0x) -> {
         return var0x.toString().length() > 2;
      };
      Integer var3 = 22;
      System.out.println(var1.test(var3));
   }

To get this, I set the view menu to Pane 2/Fernflower/Java.

user_45675520
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